This invention relates generally to membrane gas separators, and in particular to a system and method using a membrane gas separator for separating a portion of a gas from an intermittently supplied feed.
Membrane gas separators are commonly used to remove a portion of a first gas from a gaseous feed to produce a desired product. For example, the PRISM.RTM. CACTUS.RTM. dryer, manufactured by Permea, Inc., St. Louis, Mo., removes water vapor from a moist air feed to produce a drier air product. In this type of dryer, moist air is fed through a bundle of tubular membranes. The membranes are porous so that some of the air permeates through the sidewalls of the membranes; the balance of the feed passes through the membranes without permeating the sidewalls. Because of the materials selected for the membranes and the way the membranes are constructed, water vapor permeates through the sidewall at a faster rate than do the other gases of the moist air feed. Because water vapor is more quickly permeated than the other gases, the portion of the feed which does not permeate through the membranes, i.e., the nonpermeate product, has a lower concentration of water vapor than does the feed.
This type of separator is quite effective during continuous (steady state) operation. However, the operation of this type of separator when the feed is supplied intermittently is often not as effective. The inventors have discovered that this may be due to the fact that some residual water vapor remains in the separator when flow through the separator is stopped. When the flow is restarted, the residual water vapor flows out with the nonpermeate product rather than permeating the membrane sidewalls. Thus, the nonpermeate product exiting the separator during restart is not as dry as product exiting the separator during steady state operation. Therefore, in a system in which feed to the membrane separator is frequently cycled on and off, the membrane separator loses effectiveness.